[execpol.objects] Add index for unseq (cplusplus#8488)

Author: YexuanXiao

source/algorithms.tex

@@ -781,9 +781,11 @@
 \indexlibraryglobal{seq}%
 \indexlibraryglobal{par}%
 \indexlibraryglobal{par_unseq}%
+\indexlibraryglobal{unseq}%
 \indexlibrarymember{execution}{seq}%
 \indexlibrarymember{execution}{par}%
 \indexlibrarymember{execution}{par_unseq}%
+\indexlibrarymember{execution}{unseq}%
 \begin{itemdecl}
 inline constexpr execution::sequenced_policy            execution::seq{ @\unspec@ };
 inline constexpr execution::parallel_policy             execution::par{ @\unspec@ };